Circumferential grooves and other features such as shoulders and beads may be formed in pipe elements by various methods, one of particular interest being roll grooving. Roll grooving methods involve engaging an inner roller with an inner surface of a pipe element and an outer roller with an outer surface of the pipe element opposite to the inner roller and incrementally compressing the sidewall of the pipe element between the rollers while rotating at least one of the rollers. Rotation of one roller (often the inner roller) causes relative rotation between the roller set and the pipe element, and features on the inner and outer rollers form corresponding features on the inner and outer surfaces of the pipe element. In one example roll grooving method the rollers remain in a fixed location and the pipe element rotates about its longitudinal axis relative to the rollers. In another example embodiment the pipe element remains stationary and the roller set traverses the pipe element's circumference.
During roll forming of a circumferential groove in a pipe element for example, it is important to maintain the pipe element engaged with the rollers. As disclosed in U.S. Pat. No. 5,279,143 (hereby incorporated by reference) and shown in FIG. 1 herein, it is observed that, for clockwise rotation of the inner roller (when viewed along the line of sight axis defined by arrow 9), if an orientation angle 10 in a vertical plane is maintained between the longitudinal axis 12 of a pipe element 14 and the rotational axis 16 of the inner roller 18, the pipe element 14 will be forced inwardly toward the rollers (to the left in FIG. 1) such that it will abut and remain in contact with a flange 20 extending outwardly from the inner roller 18. Orientation angles 10 of from about 1°-2° provide adequate tracking force to keep the pipe element 14 engaged with the rollers. If the orientation angle 10 reverses, forces on the pipe element 14 reverse and the pipe element will tend to spiral away from the rollers and disengage therefrom.
FIG. 1 shows a side view of the pipe element and rollers, depicting the orientation angle 10 in the vertical plane; there is however a similar tracking issue that is affected by orientation angles in the horizontal plane, shown in FIG. 2. FIG. 2 shows the pipe element 14 (in broken line) and the inner roller 18 from above (the outer roller not shown for clarity). An orientation angle 22 between the longitudinal axis 12 of the pipe element 14 and the rotational axis 16 of the inner roller 18, skewed to the left in the horizontal plane, is also shown. For rotation of the inner roller 18 clockwise (when viewed along the line of sight axis defined by arrow 9), a left skewed orientation angle, if too large (generally in excess of about 2°), has disadvantages, as the pipe may overtrack aggressively toward the flange 20. Aggressive overtracking causes friction between the pipe end and the flange resulting in the pipe material being sheared off the end face of the pipe as the pipe is forced against the flange. In contrast, an orientation angle 22 skewed to the right (shown in FIG. 3) results in a reversal of forces on the pipe element 14 which cause it to spiral out of engagement with the rollers. While an orientation angle of exactly zero (axes 12 and 16 aligned in the horizontal plane, not shown) provides adequate tracking and minimizes contact between the pipe end and the flange (thereby minimizing the adverse effects of friction) it is not always possible to ensure and/or maintain an orientation angle of exactly zero in the horizontal plane. Furthermore, it is advantageous to avoid a right skewed orientation angle (shown in FIG. 3) to prevent pipe element disengagement. Thus, while a left skewed orientation angle may have disadvantages if too large, it is preferred over a right skewed orientation angle, and provides a margin of acceptability against pipe element disengagement over the zero orientation angle. There is clearly a need for an improved inner roller that can mitigate the adverse effects of aggressive tracking so that a broader range of left skewed orientation angles in the horizontal plane may be used to ensure that the pipe element tracks toward the flange and maintains proper engagement with the rollers during roll forming. (Note that for counterclockwise rotation of the inner roller 18 when viewed along line of sight 9, the conditions are reversed and a right skewed orientation angle as shown in FIG. 3 provides the desired tracking of the pipe element toward the flange 20.)